LINEAR VS ANGULAR PHENYLENES - AN INTERPLAY OF AROMATICITY, ANTIAROMATICITY, AND BAEYER STRAIN IN FUSED MOLECULAR-SYSTEMS

Geometric structure and electronic properties of some first [N]phenylenes, where N stands for the number of benzene fragments, are examined by the HF/6-31G* and MP2(fc)6-31G*//HF/6-31G* theoretical methods. It is found that angular isomers are slightly more stable than their linear counterparts. The difference increases with N. The fundamental importance of the biphenylene bonding pattern for the stability of higher [N]phenylenes is established. It appears, somewhat paradoxically, that the lower total energies of the angular [N]phenylenes are a consequence of the fact that the electronic structure is more localized in the angular than in the linear [N]phenylenes. A plausible explanation is given in terms of decreased antiaromatic character of the planar four-membered rings and by the synergism between sigma and pi electrons, in contrast to some antagonism in the linear [N]phenylenes. Simple indices of localization based on the CC bond distances and/or pi-bond orders are introduced, which offer an interesting insight into aromaticity defects in particular benzene rings. Intrinsic destabilization energies E(d) (generalized strain) are estimated by homodesmic chemical reactions. It is found that E(d) values follow an approximate but simple additivity rule being determined by the number of cyclobutadiene moieties. The calculated structural parameters are in good agreement with the available experimental data. Both are in accordance with the significant Mills-Nixon effect in angular phenylenes.